Such aircrafts are equipped with a pressurized cabin. The pressure value of the cabin is often referred to as the cabin altitude. Cabin altitude is defined as the altitude corresponding to the pressurized atmosphere maintained within the cabin. This value differs from the aircraft altitude which is its actual physical altitude. Correspondence between pressure and conventional altitude are defined in tables.
The aircrafts are further equipped with a breathing system to deliver a breathable gas to all passengers and crew members in emergency situations such as a cabin pressurization accident or smoke in the cockpit. The breathing system includes pressurized oxygen cylinders, chemical generators, or On-Board Oxygen Generator System (OBOGS) or more generally any sources of oxygen.
The breathing system further comprises breathing masks and a regulator. Usual oxygen regulators include at least a dilution mode, in which the equipment delivers a breathing mixture of breathing oxygen and diluter gas (air), the concentration of the breathing oxygen depending on the cabin altitude.
Furthermore, the regulator may be able to supply the required respiratory gas volume according to the wearer's demand.
In the event of a depressurization, the cabin altitude increases and tends to converge towards the aircraft altitude. In the same time, the pilot reduces the aircraft altitude in order to minimise the time during which the cabin altitude is too high to enable passengers who have not correctly donned a breathing mask to survive without after-effects.
Then, the pilot may slightly increase the aircraft altitude to a diversion altitude in order to improve the operating conditions of the engine and to reach a diversion airport.
The minimum concentration of oxygen in the breathing mixture is set for civil aviation by the Federal Aviation Regulations (FAR). The minimum oxygen concentration for crew demand equipments as requested by the FAR is shown in particular in document WO2007/121773.
Most of the current crew breathing masks are equipped with oxygen regulators using pneumatic technology to satisfy to this requirement. In this technology, ambient air is sucked by a Venturi made through high velocity oxygen flow. An aneroid capsule (called also altimeter capsule) regulates the altimetric oxygen enrichment by adjusting the section of the ambient air inlet.